This invention relates to mercury dosing of electrical discharge devices and, more particularly, to an improved mercury vapor generating composition and assembly which rapidly releases mercury vapor when the composition is elevated to a predetermined temperature.
A variety of electrical discharge devices, including mercury vapor rectifiers, cold cathode display devices, mercury arc lamps, and fluorescent lamps, contain fill gases in which mercury vapor is a key component. The mercury is introduced into the lamp or the like during manufacture. Liquid mercury, for example, can be introduced directly into a lamp during the exhaust cycle which occurs after the high temperature bake-out cycle of the discharge lamp is completed. However, this technique has several disadvantages. Control over the quantity of mercury introduced into the lamp is poor due to evaporation and exhaust during the cycle. Therefore, excess mercury, typically 2 to 3 times the required amount, is introduced into the lamp to ensure that a sufficient residual quantity remains. The mercury which escapes from the lamp during processing not only necessitates frequent cleaning of the vacuum system but also poses a health hazard to the operators of the vacuum system.
In another approach to mercury dosing, a glass or metal capsule containing a measured quantity of mercury is sealed within the discharge lamp. The mercury is released by thermal breaking of the capsule after the lamp is made. Although mercury vapors are reduced in the lamp production area, the use of the mercury containing capsule is not entirely satisfactory for other reasons.
A third approach to mercury dosing of electrical discharge devices utilizes mercury-containing intermetallic compounds which are sufficiently stable to withstand a discharge lamp bake-out cycle of about 600.degree. C. yet which release mercury at a predetermined temperature above that of the bake-out cycle. The mercury-releasing composition is sealed into the discharge lamp and then is heated to release the mercury vapor. A mercury-releasing device containing an intermetallic compound of mercury with titanium or zirconium is disclosed in U.S. Pat. No. 3,657,589, issued Apr. 18, 1972 to Della Porta et al. The disclosed compounds, including Ti.sub.3 Hg, Zr.sub.3 Hg and mixed compounds such as Zr.sub.2 TiHg, are sufficiently stable to permit high temperature outgassing of a discharge lamp at 500.degree. C., lamp sealing and subsequent mercury emission at 550.degree. C.-950.degree. C. The time required to dispense all of the mercury depends on the temperature to which the composition is heated because the rate of mercury emission is dependent upon its diffusion out of the solid intermetallic compound. The disclosed compositions typically require 25-30 seconds at temperatures over 900.degree. C. for suitable mercury vapor emission. Since fluorescent lamps are typically processed on a production line at a rate of one per second, an emission time of 30 seconds necessitates simultaneous heating of at least 30 lamps.
It is therefore an object of the invention to provide a mercury-releasing device and compound which has a low mercury vapor pressure up to 600.degree. C. A further object is to provide a compound which rapidly releases mercury at a predetermined temperature between 770.degree. C. and 1280.degree. C. In releasing device and compound which does not release gases which would contaminate the discharge device when heated to release mercury.